Police said more than 20 people were wounded and the death toll could rise. The imam of the mosque, 25 km (15 miles) from the southwestern city of Quetta, was among those killed, police said.
There was no immediate claim of responsibility for the blast which comes as the Taliban and the United States are in the final stages of negotiations on an agreement letting America end its longest war and withdraw its troops from neighbouring Afghanistan.
Taliban leader Haibatullah Akhundzada was not in the mosque when the bomb went off but his younger brother, Hafiz Ahmadullah, was among those killed. The Taliban leader's son was wounded, one of the sources said.
The mosque was known to be visited by members of the Afghan Taliban, the sources said.
Pakistani police did not confirm the identity of any of the victims.
"It was a timed device planted under the wooden chair of the prayer leader," said Abdul Razzaq Cheema, chief of police in Quetta, capital of Balochistan province.
One of the sources, who visited the site after the blast, said security at the mosque was always very tight.
Separatist insurgents and Islamist militants are active in Balochistan but many in Pakistan are likely to suspect the Afghan government, which is battling the Taliban over the border in Afghanistan, of the blast.
Pakistan also accuses India of meddling in Balochistan. India denies that.
Prospects of peace
Pakistan has promised to help the United States end the Afghan war and both US and Taliban negotiators have recently reported significant progress in their talks in Qatar.
But Friday's blast will raise concerns about prospects for peace.
The Taliban, in exchange for a US troop withdrawal, are expected to guarantee that Afghanistan will not be used for international terrorism.
The militants, fighting to expel foreign forces and set up an Islamic state, are also expected to make a commitment to power-sharing talks with the US-backed government and a ceasefire.
US President Donald Trump is keen to bring the troops home but many Afghans fear a US troop withdrawal will leave the government battling the Taliban alone.
The United States has about 14,000 troops in Afghanistan, with a focus on training Afghan forces and counter-terrorism.
The Taliban roam through more territory now than they have since their 2001 ouster and fighting between government forces and the insurgents has been heavy.
Police said more than 20 people were wounded and the death toll could rise.